This is a well proven Hadith among Sunni fellows, Imam Tirmidhi graded as Sahih hadith, Bukhari also wrote that the Holy Prophet (pbuh) said a rebellious group will kill Ammar(ra), Ammar will invite them towards heaven but the group will invite Ammar bin Yasir towards hell. We know that Ammar bin Yasir (ra) got killed fighting against Moawiya's army.

We can find in these narrations that Moawiya bin Abu Sufyan was happy on the killing of Hazrat Ammar bin Yasir that's why those two men were getting proud in front of Moawiya to get prize from Moawiya. We can also find in other narration that Moawiya gave wrong interpretation on killing of Ammar bin Yasir and said those people are responsible for killing of Ammar who brought him to fight with my people.

English Translation:

Hanzala bin Khawalid narrated:

I was sitting with Muawiya. Two people were fighting over the head of Ammar bin Yassar. Each one of them was claiming that “I killed Ammar.” Then Abdullah bin Amrow said “Each one of you is getting happy over the killing of this person, surely I heard from the Prophet saying this, Oh Ammar the rebellious group will martyr you.”

Another Narration

English Translation:

Abdullah bin Harith narrated:

Myself, Abdullah bin Amrow, Amrow bin Alas and Muawiya were walking. Abdullah bin Amrow said “I heard from the Prophet saying this this, Oh Ammar a rebellious group will martyr you.” Amrow said “Oh Muawiya, listen what he is saying.” He(Muawiya) grabbed him and said “Did we kill Ammar? That person killed him who brought him in the war. You said wrong.”